iTextSharp 7 object reference not set to an instance of an object

问题

Is there some recommendation to build tables with cells having paragraphs in order to avoid an exception at adding of some cell to table or table to document? I get this and I can't figure out what happens:

[NullReferenceException: Object reference not set to an instance of an object.]
   iText.Layout.Renderer.TableRenderer.DrawBorders(DrawContext drawContext) +2493
   iText.Layout.Renderer.TableRenderer.DrawChildren(DrawContext drawContext) +1497
   iText.Layout.Renderer.AbstractRenderer.Draw(DrawContext drawContext) +153
   iText.Layout.Renderer.TableRenderer.Draw(DrawContext drawContext) +637
   iText.Layout.Renderer.AbstractRenderer.DrawChildren(DrawContext drawContext) +104
   iText.Layout.Renderer.BlockRenderer.Draw(DrawContext drawContext) +525
   iText.Layout.Renderer.TableRenderer.DrawChildren(DrawContext drawContext) +1382
   iText.Layout.Renderer.AbstractRenderer.Draw(DrawContext drawContext) +153
   iText.Layout.Renderer.TableRenderer.Draw(DrawContext drawContext) +637
   iText.Layout.Renderer.DocumentRenderer.FlushSingleRenderer(IRenderer resultRenderer) +473
   iText.Layout.Renderer.RootRenderer.AddChild(IRenderer renderer) +1999
   iText.Layout.RootElement`1.Add(BlockElement`1 element) +92
   iText.Layout.Document.Add(BlockElement`1 element) +81

Here is a simple snapshot (compared to the real project) using a Windows console project:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using iText.Layout;
using iText.Layout.Borders;
using iText.Layout.Element;

namespace iTextTest
{
    public static class iTextSharpHelper
    {
        public static T SetBorderEx<T>(this ElementPropertyContainer<T> element, Border border)
           where T : ElementPropertyContainer<T>
        {
            element.SetBorder(border);

            return (T)element;
        }

        public static Paragraph Style(this BlockElement<Paragraph> element)
        {
            element
                .SetBorderEx(iText.Layout.Borders.Border.NO_BORDER)
                .SetFont(iText.Kernel.Font.PdfFontFactory.CreateFont(iText.IO.Font.FontConstants.HELVETICA))
                .SetFontSize(10.0f)
                .SetFixedLeading(12.0f)
                .SetVerticalAlignment(iText.Layout.Properties.VerticalAlignment.BOTTOM)
                .SetMargin(0f);

            return (Paragraph)element;
        }
    }

    class Program
    {
        private static float[] tableColumns = { 0.35f, 0.25f, 0.15f, 0.25f };

        static void Main(string[] args)
        {
            iText.Kernel.Pdf.PdfDocument pdf = new iText.Kernel.Pdf.PdfDocument(new iText.Kernel.Pdf.PdfWriter("test.pdf"));

            iText.Layout.Document document = new iText.Layout.Document(pdf, iText.Kernel.Geom.PageSize.A4);

            document.SetMargins(50f, 50f, 25f, 50f);

            iText.Layout.Element.Table mainTable = new iText.Layout.Element.Table(tableColumns)
                .SetBorderEx(iText.Layout.Borders.Border.NO_BORDER)
                .SetWidthPercent(100)
                .SetHorizontalAlignment(iText.Layout.Properties.HorizontalAlignment.LEFT)
                .SetPadding(0f);

            for (int i = 0; i < 10; i++)
            {
                AddRow(mainTable, "ABCDEFGHIJ", "ABCDEFGHIJ", "ABCDEFGHIJ");
            }

            document.Add(mainTable);

            document.Close();
        }

        private static void AddRow(iText.Layout.Element.Table table, string col1, string col2, string col3)
        {
            // Label
            AddCell(table, col1, true)
                .SetBorderTop(new iText.Layout.Borders.SolidBorder(iText.Kernel.Colors.Color.BLACK, 0.5f));

            // Product - Voucher and price/pcs        
            AddCell(table, col2, true)
                .SetBorderTop(new iText.Layout.Borders.SolidBorder(iText.Kernel.Colors.Color.BLACK, 0.5f));

            // Message
            AddCell(table, col3, true, 2)
                .SetBorderTop(new iText.Layout.Borders.SolidBorder(iText.Kernel.Colors.Color.BLACK, 0.5f))
                //.SetBorderRight(new iText.Layout.Borders.SolidBorder(iText.Kernel.Colors.Color.BLACK, 0.5f))
                .SetHorizontalAlignment(iText.Layout.Properties.HorizontalAlignment.RIGHT)
                .SetTextAlignment(iText.Layout.Properties.TextAlignment.RIGHT);
        }

        private static iText.Layout.Element.Cell AddCell(iText.Layout.Element.Table table, string text, bool setBold = false, int colSpan = 1)
        {
            iText.Layout.Element.Cell cell = new iText.Layout.Element.Cell(1, colSpan)
                .SetBorderEx(iText.Layout.Borders.Border.NO_BORDER)
                .SetVerticalAlignment(iText.Layout.Properties.VerticalAlignment.BOTTOM);

            if (!string.IsNullOrEmpty(text))
            {
            iText.Layout.Element.Paragraph paragraph = new iText.Layout.Element.Paragraph(text)
               .Style();

            if (setBold)
                paragraph.SetBold();

            cell.Add(paragraph);
            }

            table.AddCell(cell);

            return cell;
        }
    }
}

Note, a commented out line of code:

//.SetBorderRight(new iText.Layout.Borders.SolidBorder(iText.Kernel.Colors.Color.BLACK, 0.5f))

Adding it serves as a workaround to make the document render without the exception.

回答1:

Given the sample code added by the OP the issue can easily be reproduced.

Furthermore after porting the code to iText/Java the issue could be reproduced there, too, cf. MikesTableIssue.java test method testMikesCode. Thus, it is no porting error from Java (the original iText code) to C#.

The sample could even be considerably simplified and still reproduce the issue:

try (   FileOutputStream target = new FileOutputStream("mikesTableIssueSimple.pdf");
        PdfWriter pdfWriter = new PdfWriter(target);
        PdfDocument pdfDocument = new PdfDocument(pdfWriter)    )
{
    Document document = new Document(pdfDocument);
    Table mainTable = new Table(1);
    Cell cell = new Cell()
            .setBorder(Border.NO_BORDER)
            //.setBorderRight(new SolidBorder(Color.BLACK, 0.5f))
            .setBorderTop(new SolidBorder(Color.BLACK, 0.5f));
    cell.add("TESCHTINK");
    mainTable.addCell(cell);
    document.add(mainTable);
}

(MikesTableIssue.java test method testSimplified)

The issue does not occur if one

• removes setBorder(Border.NO_BORDER) or
• removes setBorderTop(new SolidBorder(Color.BLACK, 0.5f)) or
• adds setBorderRight(new SolidBorder(Color.BLACK, 0.5f)).

In this situation com.itextpdf.layout.renderer.TableRenderer.drawBorders(DrawContext) executes this code:

if (lastBorder != null) {
    if (verticalBorders.get(j).size() > 0) {
        if (i == 0) {
            x2 += verticalBorders.get(j).get(i).getWidth() / 2;
        } else if(i == horizontalBorders.size() - 1 && verticalBorders.get(j).size() >= i - 1 && verticalBorders.get(j).get(i - 1) != null) {
            x2 += verticalBorders.get(j).get(i - 1).getWidth() / 2;
        }
    }

    lastBorder.drawCellBorder(drawContext.getCanvas(), x1, y1, x2, y1);
}

while lastBorder is the SolidBorder instance, verticalBorders is [[null], [null]], j == 1 and i == 0.

Thus, some additional null checks ought to be introduced here.

来源：https://stackoverflow.com/questions/38663251/itextsharp-7-object-reference-not-set-to-an-instance-of-an-object